MacBook Pro 2016 15" power off unexpectedly.

Hi, my MacBook power off automatically when there is heavy usage/load (many apps open). It doesn't turn on itself. but when I plug in the charger and press power button it turns on with all the apps open (like restore). During turning it on, there is some kind of quick gray screen with some progress sign.

I think this is some kind of power management issue? My battery cycle is 343 and battery is draining so fast eversince upgraded to os 12. What could be the possible solution here? I already tried reseting PRAM, SMC.